Snippe, C.

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Material modeling for the benefit ofsuperplastic forming simulationsC. SnippeNIKHEF / University of TwenteEngineering DepartmentP.O. Box 41882, NL 1009 DB Amsterdamphone +31-(0)20-5922105, email csnippe@nikhef.nlIntroductionSuperplastic materials are characterised by a highstrain rate sensitivity and failure strain. Experimentswith the superplastic aluminium alloy ALNOVI-1have to lead to a set of stress-strain curves. To-gether with a yield criterion, and the assumption ofisotropic behaviour (which is generally true for su-perplastic materials), a material model can be con-structed to be used in SPF simulations.ObjectiveThe goal is to simulate the superplastic formingprocess, using a material model which is obtainedby material experiments.ExperimentsFigure 1 : Geometry of the undeformed and a de-formed tensile specimen.Tensile tests have been performed in order to cap-ture the uniaxial stress-strain behaviour at variousstrain rates. All the tests have been carried out atthe optimal temperature for ALNOVI-1 to behavesuperplastically (525 oC). Figure 1 shows somegeometric details. Stress-strain curves at differentstrain rates can be constructed from these tests.0.0 0.5 1.0 1.5 2.0 2.5 0 5101520rate = 5E−4 /srate = 1E−3 /srate = 2E−3 /srate = 3E−3 /srate = 5E−3 /srate = 1E−2 /sNominal strain [-]True stress [MPa]Figure 2 : ALNOVI-1: stress-strain curves.Material modelThe material model is divided into three parts. Thefirst part (1) describes the initial flow stress as afunction of the strain rate, which is in case of su-perplastic materials a sigmoidal curve. Four pa-rameters may fit this curve to the experimentallyfound values. The second part (2) adds a harden-ing behaviour, due to grain growth, dependent onthe strain and the strain rate. The third part (3) addssoftening behaviour due to cavity formation.log(σy) =1a + b exp(c log ε̇)+ d (1)σd =(p ε̇qεrdref− d0 + 1)kσy (2)σcav = σd(1− n1ξn2)n3 (3)Results and future workFrom the simulation of a bulge forming process, theoptimal pressure-time curve can be constructed.Figure 3 shows this curve, together with some val-ues of the material parameters. The experimentsalso have to show the coalescense behaviour ofcavities, resulting in unwanted presence of leaks.10 40 70 100 130  0 0.4 0.81.2Time [s]Pressure [bar]Sheet thickness: 0.5 mmSheet diameter: 100 mma = 1.157b = 0.207c = -3.052d = 0.266Convergence problemsbecause of strain ratesoftening(Avg: 75%)SNEG, (fraction = −1.0)S, Mises+7.866e+06+8.229e+06+8.592e+06+8.955e+06+9.318e+06+9.681e+06+1.004e+07+1.041e+07+1.077e+07+1.113e+07+1.149e+07+1.186e+07+1.222e+07Figure 3 : Free bulging pressure-time curve.AcknowledgementsThis project is a cooperation of the University of Twente and theNational Institute of Nuclear Physics and High-Energy Physics(NIKHEF) in Amsterdam

Material modeling for the benefit of superplastic forming simulations

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